Industrial bread dough mixing often involves a period of mixing under high headspace pressure to enhance oxygen availability, followed by a period of partial vacuum to favourably manipulate the final bubble size distribution. This paper presents the results of a study using X-ray tomography to measure the gas bubble size distribution in dough samples over the course of a pressure step-change mix. The first objective of the current work was to measure bubble size distributions at points throughout a pressure-step dough mixing process using a non-synchrotron X-ray source. The second objective was to fit a simplified population balance model to the measured size distributions. The third objective was to use the data set and fitted model to explore the validity of the assumptions within the simplified model and to consolidate understanding of underlying aeration and mixing phenomena and the resultant process dynamics. It was found that the dynamic changes in the bubble size distribution of a bread dough during pressure-step mixing could be accurately measured using a laboratory X-ray source. The response of the cumulative dough voidage to a pressure-step change during mixing could be reproduced very well using the simplified population balance model (which assumes: all entrained bubbles are the same size, no bubble break-up or coalescence, and likelihood of bubble disentrainment is proportional to bubble volume). The measured response of the bubble number density and mean volume agreed reasonably well with that predicted by the simplified model (with parameters fitted to only cumulative voidage data). It is demonstrated that the decrease in number density following a pressure step-decrease is much more short-lived than the decrease in size which is permanent.